I will be playing this game with a group and we have just few hours hours to learn/teach the game and then hopefully one good mission with the most fun objectives, etc. hopefully providing the most "complete" and fulfilling experience.

Just play the first mission - it favours the marines somewhat but that always helps when you're trying to encourage others to play. It should also go fairly quickly which would leave you time to play an additional mission if you have more than a couple of hours.

My answer is a mix of both previous answers (kinda). I can't recall the exact name of this mission, but there's an assault mission towards the back of the blue mission book that uses a Priority Escort objective card (I wanna say that it's the second-to-last mission in the book, but I'm not 100% sure). It uses the same exact map layout as the first mission, but it involves the marines pushing a payload from one end of the map to the other.

I enjoy this mission because of its emphasis on strategically managing the invader's threat token supply to open portals and summon he demon horde. The real powerhouse demons can't be summoned until you get a red portal open, so the stakes are high if the invader doesn't get his first frag before the payload gets too far along.

The best invader strategy that I've come up with is to spend all of the threat tokens on rounds 1 & 2 to open all blue & yellow portals ONLY; don't summon demons until round 3. At the end of round 2's setup phase, you should have 1 threat token left over. Save it for round 3!

Also, be sure to discard 3 event cards each round so that you've got 6 argent power tokens sitting on your summoning card at the start of round 3.

Once round 3's setup phase begins, spend only 2 threat tokens to summon 3 Imps & 2 Possessed Soldiers from a yellow portal somewhere ahead of the Marines (make sure there's a wall between them and the nearest Marine, ESPECIALLY if he's got the chainsaw!!!).

Each round after round 3 should involve spending no more than 2 threat tokens. Always keep 1 threat token in your possession until you get your first frag! The idea is that you'll have 3 threat tokens at the beginning of each round; once you manage to kill your first Marine, summon something from a freshly-opened red portal!!!

If the payload is within 6 spaces of its final destination, summon a Cyberdemon and make him stand directly on top of the payload to make it difficult for the Marines to stand close enough to push the payload at the end of the round (Rail Cannon anyone who dares to try!). If the cart is much further away from its destination, then bring in the Baron of Hell and his shit-kickin' squad. After summoning from a red portal, try to get back to always having 1 threat token on-hand for the next red portal summoning!

I love this map a lot, and it's much more balanced than the first mission. I hope you give it a try and enjoy it as much as I do!